Category: Colloquia

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Geology Colloquium

Wed Feb 29, 2012. 03:30 pm - 05:00 pm
Benson 180

Imaging with Geophysics: The Heat and Water Cycling in Modern Subduction Zones

Geoff Albers, Lamont Doherty Earth Observatory, Columbia University GeoPRISMS Distinguished Lecture Program

Abstract. The thermal structure of subduction zones controls many first-order phenomena: the depth extent of earthquakes, dehydration and potentially melting of the subducting plate, the onset and character of arc volcanism, the flux of volatiles into the deep earth, and ultimately the buoyancy forces driving plate motions. Increasingly, evidence suggests that most of these phenomena are regulated by the presence and geometry of a “cold nose”, a region of the mantle wedge trenchward of the arc that is cold compared to flowing asthenosphere beneath the arc, and probably decoupled from large-scale flow. While the presence of this cold nose has been inferred for some time, only recently are enough critical observations emerging to confirm that this is a general property of subduction zones. While the physical cause of this stagnant zone remains unclear, it likely reflects mechanical decoupling along the plate interface to much greater depths (70-90 km) than inferred from the down-dip limit of seismicity on the thrust zone (25-45 km). Much of the evidence for the cold nose comes from heat flow and, increasingly, measurements of seismic attenuation. Also, interfaces mapped by seismic mode conversions place clear constraints on the metamorphism of subducting oceanic crust. They show major eclogite formation and dehydration at depth that varies with subduction parameters, consistent with seismic velocities varying with dehydration reactions. At depths of 70 – 90 km the onset of coupling drags hot mantle into the wedge and temperatures rapidly increase to those necessary to generate basaltic arc magmas (>1200-1300°C in the wedge). Hence, the location of the decoupling transition may also control the location of volcanic arcs in subduction zones; the arc front occurs at the most trenchward place where the mantle is hot enough to melt. Similarly, major dehydration of oceanic crust probably occurs rapidly once the subducting plate passes the decoupling transition.

Figure Caption: Click Here for Figure Three seismic images of the Nicaragua Subduction Factory. (A) 2D receiver function migration, showing S-wave velocity variations needed to generate scattered P coda, such as Moho and top of subducting plate. (B) S-wave attenuation as 1000/Qs, from tomographic inversion, at 1 Hz. 1/Qs responds to temperature and indicates high-temperature region beneath and behind arc. (C) Vp/Vs anomalies from regional travel-time tomography. In regions of high temperature, high Vp/Vs may indicate presence of melt, perhaps showing a vertical melt column beneath volcanic arc.

Refreshments served at 3:30 on the 3rd floor.

All are welcome.

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